1. Field of the Invention
This invention relates to protective layers applied to optical coatings on transparent substrates. In particular, the invention relates to the use of a temporary protective layer of carbon. In addition, the invention relates to a scratch propagation blocker (SPB) protective layer applied to the outermost layer of various optical coatings.
2. Discussion of the Background
Optical coatings are deposited on transparent substrates to reflect or otherwise alter the transmission of some or all of the radiation incident on the substrates. For example, the optical coating of a mirror is designed to reflect visible light. Low-emissivity optical coatings are designed to reduce the transmission of infrared radiation. Optical coatings generally include two or more different layers each having a thickness in a range of from less than 1 nm to over 500 nm.
Optical coatings are frequently damaged during shipping and handling by scratching and by exposure to corrosive environments. Silver based low-emissivity coatings in particular have been plagued with corrosion problems since their introduction into the fenestration marketplace decades ago. Attempts at improving the durability of optical coatings have included the application of a temporary protective layer such as a plastic adhesive backed film. Other protective layers have been formed by applying and curing solvent based polymers on glass.
However, a number of problems are associated with using adhesive films and polymer films as protective layers on optical coatings. Expensive, specialized equipment is required to apply the adhesive films and the polymer films to optical coatings. When an adhesive film is pulled away from an optical coating, the adhesive film runs the risk of removing portions of the optical coating. Even if portions of the optical coating are not removed, the force on the optical coating associated with removing the adhesive film can damage the optical coating. A solvent based polymer film applied to an optical coating must be dried and the solvent removed in an environmentally friendly manner. Removal of the polymer film from an optical coating requires specialized washing that can easily damage the optical coating.
For protection from corrosion, most silver based low-emissivity stacks in use today make use of barrier or cladding layers in direct contact and on one or both sides of the silver layers. It is well known in the art that various thin film layers can function as barriers to movement of corrosive fluids such as water vapor and oxygen. Metals layers are known to be particularly effective diffusion barriers due to their ability to physically and chemically inhibit diffusion of corrosive fluids. Metal layers tend to be more effective physical barriers to diffusion than dielectric layers such as oxides, because both evaporated and sputtered metal layers tend to contain fewer pinhole defects than oxide layers. Metal layers also tend to chemically block diffusion by reacting with fluids diffusing through a pinhole to stop the movement of all chemically bound fluid molecules. The bound fluid molecules in turn restrict the passage of additional fluid through the pinhole. The more reactive metals are particularly effective for chemically blocking.
Tempering greatly reduces the corrosion problems associated with silver based low-emissivity coatings. Tempering results in an atomic level restructuring to a lower energy state and renders the silver far less prone to corrosion. Tempering also improves the hardness and scratch resistance of optical coatings.
However, until optical coatings are tempered, the coatings remain particularly susceptible to damage from scratching and corrosion. Even after tempering, optical coatings are not immune from scratching and corrosion.
Scratches in an optical coating frequently do not become visible until after the coating is heated and tempered, which can cause the scratches to grow and propagate.
Carbon has been used as a protective coating on glass substrates. For example, U.S. Pat. No. 6,303,226 discloses the use of an amorphous, diamond-like carbon (DLC), protective layer on a glass substrate.
There is a need for improved methods and layers for protecting optical coatings.